The present invention relates to defrost control in a refrigerator, and more particularly to an adaptive refrigerator defrost control.
Defrost operations are performed in refrigerators to clean the evaporator coils and to keep the coils free from frost build up. Known defrost controls for refrigerators typically are based on fixed timer controls. More specifically, and with such known controls, a defrost operation is initiated after a fixed amount of compressor run time. After the defrost operation is initiated, the control keeps the compressor off for a second fixed period of time. Defrost heat is terminated during the second fixed time period by a thermal sensing device for sensing the temperature of the evaporator coils.
The compressor run time is affected, for example, by the length of time that the fresh food and freezer doors are open. Particularly, if the doors are often open and the compartments warm, the compressor runs more than if the doors are not opened very often and the compartments remain relatively cool. Although warming of the compartments may require that the compressor run more, such warming does not necessarily require that defrost operations be performed more often. Initiating defrost operations after a fixed amount of compressor run time, however, results in performing defrost operations more often. Unnecessarily performing defrost operations results in increased, and unnecessary, energy consumption.
In addition, during defrost operations, the temperature in the freezer compartment generally is not allowed to exceed a predetermined peak temperature, e.g., about 0.5xc2x0 F. To enable completion of the defrost operation without exceeding the peak temperature, the freezer compartment normally is maintained at about xe2x88x927xc2x0 F., which is cooler than required for normal operations but necessary to prevent excessive warming during defrost operations. If the freezer temperature during normal operations could be increased even just a few degrees without resulting in exceeding the predetermined peak temperature during defrost operations, a potentially significant energy savings could be provided.
In an exemplary embodiment of the invention, an adaptive defrost control includes a microcomputer for controlling the initiation and termination of a defrost operation based, at least in part, on opening of the fresh food door and the freezer door, as well as the state of a defrost heater and the compressor. The adaptive defrost control monitors both the compressor run time and the fresh food and freezer door open times. For initiating defrost, a timer of the control counts toward a Maximum Time Till Defrost, which is the sum of (i) the compressor run time, (ii) the fresh food door open time multiplied by a Fresh Food Rate (negative), and (iii) the freezer door open time multiplied by a Freezer Rate (negative). Therefore, rather than a fixed period of time before initiation of a defrost operation, the adaptive control provides that the time until defrost is adjusted based on opening of the fresh food and freezer doors.
Once initiated, and during a defrost operation, the adaptive control operates the refrigerator in a pre-chill state of a fixed period of time. The pre-chill state is provided so that the freezer is cooled prior to applying heat to the evaporator coils so that the defrost heat will have less of an affect on the food temperature and on the maximum temperature after defrost. Therefore, when the control has determined that a defrost operation should be initiated, the heating portion of a defrost is preceded by a Pre-Chill Time when the compressor is held on without regard to a cold control demand for cooling. The pre-chill operation enables normally maintaining the freezer compartment at a temperature a few degrees higher than with known refrigerators, which provides an energy savings. After pre-chill, the defrost heater is energized to clear the evaporator coils of ice.
Bimetal switches (sometimes referred to herein as terminators) are electrically connected in series with the defrost heater. The switches are responsive to the heat from the defrost heater after the coils have been cleared of frost and ice. After the control has sensed that the terminators have operated, i.e., opened, the control initiates a dwell.
Dwell is the time period after defrost heat is terminated and before the compressor is allowed to turn back on, i.e., before the cold control re-energizes the compressor. Although dwell time preferably is minimized, sufficient time must be provided to allow the freon pressures to equalize so that the compressor properly operates and to allow water to drip off the evaporator. To minimize dwell time, the termination of defrost heat by the external bimetal switches (i.e., the terminators) is monitored. Once the bimetal switches terminate the heating, dwell time is entered and the control holds the compressor off until the dwell time is ended. Once the defrost sequence is complete, the cold control then re-energizes the compressor while the adaptive control monitors the compressor on time. The adaptive control continues the monitoring function to determine when to reenter the defrost sequence.
If the defrost heater on time, i.e., the time from initiation of defrost operations to opening of the terminators, is longer than expected, the defrost is terminated by the adaptive control. When the control terminates a defrost, e.g., defrost time exceeds a Defrost Heat Time, a defrost relay is opened and the cold control is re-energized. Therefore, after a time terminated defrost, there is no dwell time.
If a defrost operation requires an abnormally long time, as defined by an Abnormal Defrost Delta Time, or has terminated due to a Defrost Heat Time, then the adaptive controller determines a time for a next defrost operation based only on the compressor run time with no door open adaptive features. This next defrost occurs after the compressor has run a fixed compressor run time referred to as an Abnormal Run Time. If a defrost operation is terminated by the Defrost Heat Time, initiation of the next defrost is determined by the Abnormal Run Time.
Also during adaptive run time, there is a minimum time between defrosts to ensure that a failed switch or a door open condition does not cause unnecessary defrost operations. For example, if a refrigerator door is left open, a refrigerator may enter into defrost operations every 2 hours. By requiring that a minimum time must elapse prior to entering defrost, such excessive defrost operations are avoided.
In addition to being adaptive, the control enables immediate entry into a defrost or pre-chill state for product service or test purposes. In an exemplary embodiment, by manually depressing the fresh food light switch a preselected number (e.g., 3) times within a preselected time period (e.g., 5 seconds), an immediate (no pre-chill) defrost is initiated unless the control is already in the defrost or dwell states. Once in the defrost state, and if the fresh food light switch is depressed a preselected number (e.g., 3) times within a preselected time period (e.g., 5 seconds), then the defrost state is exited. The pre-chill state can be entered by manually depressing the fresh food light switch a different preselected number (e.g., 6) times within the preselected time period (e.g., 5 seconds).